AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

INTRODUCTION

Section 2 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Background

Polymyositis (PM) is an idiopathic inflammatory myopathy with symmetric proximal muscle weakness, elevated skeletal muscle enzyme levels, and characteristic electromyography (EMG) and muscle biopsy findings. Clinically similar to PM, dermatomyositis (DM) is an idiopathic inflammatory myopathy associated with characteristic dermatologic manifestations. Inclusion body myositis (IBM) is a slowly progressive idiopathic inflammatory myopathy generally found in older males with characteristic pathological findings. Bohan and Peter classify the idiopathic inflammatory myopathies as follows:

• I - Primary idiopathic PM
• II - Primary idiopathic DM
• III - PM or DM associated with malignancy
• IV - Childhood PM or DM
• V - PM or DM associated with another connective-tissue disease
• VI - IBM
• VII - Miscellaneous (eg, eosinophilic myositis, myositis ossificans, focal myositis, giant cell myositis)

Pathophysiology

Although the initial inciting agent remains unknown, possibilities include virus-mediated muscle injury or microvascular insult leading to release of muscle autoantigens. These autoantigens are then presented to T lymphocytes by macrophages in the muscle. Activated T lymphocytes proliferate and release cytokines such as interferon gamma (IFN-gamma) and interleukin 2 (IL-2). IFN-gamma promotes further macrophage activation and release of mediators of inflammation such as IL-1 and tumor necrosis factor-alpha (TNF-alpha).

Additionally, these cytokines induce aberrant expression of major histocompatibility complex (MHC) class I and II molecules and adhesion molecules on muscle cells. Destruction of muscle fibers occurs when CD8⁺ T lymphocytes (cytotoxic) encounter antigens in conjunction with MHC class I molecules on muscle cells. Macrophages further the process of destruction both directly and by secreting cytokines.

Frequency

United States

Idiopathic inflammatory myopathies are relatively rare diseases, and incidence ranges from 0.5-8.4 cases per million population.

International

A lower incidence among Japanese persons has been observed.

Mortality/Morbidity

Most patients with PM respond favorably to immunosuppressive therapy but may require lifelong treatment. Five-year survival rates have been estimated at more than 80%. Causes of death include severe muscle weakness, pulmonary involvement, cardiac involvement, associated malignancy, and complications of immunosuppressive therapy, especially infection.

Race

In the United States, an increased incidence in the black population has been observed.

Sex

PM affects women more frequently than men (2:1 ratio); IBM affects men twice as frequently as women.

Age

PM usually affects adults older than 20 years, and especially people aged 45-60 years. PM rarely affects children, unlike DM.

• The age of onset of PM with another collagen vascular disease is related to the associated condition.
• Eighty percent of patients with IBM are older than 50 years at onset.

CLINICAL

Section 3 of 11  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

History

Symptoms of PM gradually accrue over a period of 3-6 months. Diagnosis is usually delayed because, unlike in DM, no associated rash occurs before onset of muscle disease.

• Muscular manifestations
• • Patients usually present with symmetric proximal muscle weakness in the upper and lower extremities.
  • Weakness of neck flexors also occurs.
  • Initially, distal muscle weakness is rare, but it may occur late in the course of the disease.
  • Patients may report muscle pain and tenderness that may be confused with polymyalgia rheumatica.
  • Dysphagia secondary to oropharyngeal and esophageal involvement occurs in about a third of patients and is a poor prognostic sign. Dysphonia is also a poor prognostic sign but is much less common.
  • Ocular muscles are never involved in generalized PM. However, isolated orbital myositis, an inflammatory disorder involving the extraocular muscles, is well described.
  • Facial and bulbar muscle weakness is extremely rare.
• Constitutional manifestations: PM is a systemic disease, and patients can present with the following symptoms:
• • Morning stiffness
  • Fatigue
  • Anorexia
  • Fever (associated with anti-synthetase antibodies such as anti-Jo-1)
  • Weight loss
• Pulmonary
• • Pharyngeal and esophageal weakness may lead to aspiration pneumonia.
  • Patients may experience exertional dyspnea secondary to weakness of chest wall muscles and diaphragmatic muscles.
  • Patients receiving immunosuppressants have an increased risk of infections.
  • Interstitial lung disease occurs in 5-30% of patients with idiopathic inflammatory myopathy (associated with anti-synthetase antibodies, especially anti-Jo-1). Patients may be asymptomatic or present with exertional dyspnea, cough, and fever.
  • Interstitial pneumonitis, bronchiolitis obliterans organizing pneumonia, and pulmonary capillaritis have all been described in PM.
• Cardiac
• • Cardiac involvement is unusual and, if present, portends a bad prognosis.
  • Rhythm disturbances, conduction defects, congestive heart failure, pericarditis, pulmonary hypertension, and myocarditis can occur.
• Joint involvement
• • Patients can present with arthralgias or arthritis.
  • Arthritis is usually symmetric and involves the knees, wrists, and hands (associated with antisynthetase antibodies).
  • A severe deforming arthropathy without erosions has been reported; erosive changes are very rare.
• Overlap syndromes
• • PM can be associated with another connective-tissue disease such as systemic lupus erythematosus, rheumatoid arthritis, mixed connective-tissue disease, Sjögren syndrome, or scleroderma.
  • About 25% of patients with scleroderma have myositis; this phenomenon has been associated with anti-PM-Scl (anti-PM-1) antibody.
  • In Japan, anti-Ku antibody has been described with this condition.
• Gastrointestinal
• • Dysphagia
  • Odynophagia
  • Nasal regurgitation
  • Reflux esophagitis
  • Abdominal bloating
  • Constipation
• Kidneys: Intrinsic renal disease is rare. Occasionally, severe rhabdomyolysis with myoglobinuria can result in acute tubular necrosis.
• Skin
• • Rash is absent, unlike with DM.
  • "Mechanic's hands" (associated with antisynthetase antibodies), ie, hyperkeratotic eruptions over the finger pads and lateral aspects of the fingers, have been reported.
  • Raynaud phenomenon has been described in patients with antisynthetase antibodies.
  • Rarely, periorbital edema may occur (best described in DM).
  • Calcinosis occurs in approximately 5% of patients with PM (in association with sclerodermalike illness).
  • Telangiectasias are uncommon.
• IBM: This condition is a slowly progressive idiopathic inflammatory myopathy that mostly affects men older than 50 years. Muscle involvement includes predominantly proximal muscles but may also include distal muscles (50%), and involvement may be asymmetric. Dysphagia is found in most patients (60%).

Physical

Physical examination generally shows symmetric proximal muscle weakness.

• Deep tendon reflexes are usually preserved, and sensory examination findings are normal.
• Occasionally, muscle tenderness may be present.
• Dysphonia with nasal speech may be noted.
• Lung examination may show evidence of interstitial lung disease such as dry inspiratory crackles in the lung bases ("Velcro").
• IBM: This conditions manifests as severe proximal muscle weakness with atrophy, often with distal muscle weakness. The weakness may be asymmetric. Deep tendon reflexes may be decreased or absent if weakness is severe.

Causes

The causes of PM are still understood poorly, although the condition is believed to be an immune-mediated process triggered by environmental agents in genetically predisposed individuals. Increased association of myositis has been found with HLA haplotypes A1, B8, and DR3, which also increases the risk for autoimmune diseases. Environmental triggers, especially infectious agents, have been suggested as etiologic agents, as follows:

• Infectious agents
• • Coxsackie virus B1
  • HIV
  • Human T-lymphotropic virus 1 (HTLV-1)
  • Hepatitis B
  • Influenza
  • Echovirus
  • Adenovirus
• Noninfectious agents
• • Many drugs are known to cause myopathy. Most drugs such as hydroxychloroquine and colchicine cause a toxic or metabolic myopathy.
  • Several drugs, however, rarely induce an immune-mediated myopathy or myositis. Muscle biopsy shows chronic inflammatory changes consistent with PM. Drugs such as D-penicillamine, hydralazine, procainamide, phenytoin, and ACE inhibitors have been associated with this type of inflammatory myopathy.
  • Statins can be associated with severe muscle inflammation and rhabdomyolysis.
• Recognition of other autoimmune diseases with PM and the prevalence of circulating autoantibodies strongly favor an autoimmune etiology.

DIFFERENTIALS

Section 4 of 11  

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• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Other Problems to be Considered

• Other idiopathic inflammatory myopathies
• • IBM
  • Eosinophilic myositis
  • Myositis ossificans
  • Focal myositis
  • Giant cell myositis
• Amyotrophic lateral sclerosis
• Diabetic polyradiculopathy
• Drug-induced myopathy
• • Alcohol
  • Antimalarials
  • Clofibrate
  • Colchicine
  • Ketoconazole and other azole antifungal agents
  • Statin/3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors
  • D-penicillamine
  • Vincristine
  • Zidovudine (AZT)
• Metabolic myopathy
• Muscular dystrophy
• Myasthenia gravis
• Overlap connective-tissue diseases

WORKUP

Section 5 of 11  

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• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Lab Studies

• Serum creatine kinase (CK) levels are usually elevated from 5-50 times the normal value. Serum CK levels may be used to monitor myositis activity along with careful physical examination. However, serum CK levels can be within reference ranges despite increased disease activity (eg, in cases of chronic and late-stage PM). CK levels are usually minimally elevated or within reference ranges in patients with IBM. CK level are within reference ranges in patients with corticosteroid-induced myopathy.
• Other muscle enzymes may be elevated.
• • Lactic dehydrogenase
  • Aspartate aminotransferase
  • Alanine aminotransferase
  • Aldolase
• Nonspecific markers of inflammation include the following:
• • CBC count may show leukocytosis or thrombocytosis.
  • Elevated erythrocyte sedimentation rate or C-reactive protein occurs in 50% of patients with PM.
• Antinuclear antibody assay findings are positive in a third of patients with PM and in only 15% of patients with IBM.
• Myositis-specific antibodies are associated with PM.
• • Antisynthetase antibodies, such as anti-Jo-1 antibodies, are associated with certain clinical features. Patients with antisynthetase syndrome demonstrate idiopathic inflammatory myopathy, interstitial lung disease, arthritis, Raynaud phenomenon, fever, and mechanic's hands.
  • Antibodies to signal recognition particles (SRPs) are present in about 4% of patients with PM and are associated with acute onset of severe weakness, increased incidence of cardiac involvement, and higher mortality rates.
• Perform age-appropriate evaluation for malignancy.

Imaging Studies

• Muscle-imaging techniques such as MRI and ultrasonography may be useful to document and localize the extent of muscle involvement (see Image 1).
• Chest radiographs and high-resolution CT scans of the chest are helpful for evaluation of interstitial lung disease.
• Barium swallow studies are helpful for evaluation of dysphagia or dysphonia.

Other Tests

• EMG findings are abnormal in almost all patients (90%). Various abnormalities consistent with PM may be found, depending on the stage of disease. In IBM, both myopathic and neuropathic changes may be present.
• • Evidence of membrane irritability, increased insertional activity, fibrillation potentials, positive sharp waves at rest
  • Myopathic changes of motor unit action potential, decreased amplitude and duration, increased polyphasic potentials, bizarre high-frequency repetitive discharges
  • Chronic changes, evidence of denervation-reinnervation
• Pulmonary function tests and diffusion capacity for evaluation of interstitial lung disease may be appropriate.

Procedures

• Muscle biopsy is crucial in helping diagnose PM and in excluding other rare muscle diseases. MRI can be used to guide the site of biopsy. Avoid biopsy of sites recently studied by EMG by using the contralateral side.

Histologic Findings

Muscle biopsy shows muscle fibers in varying stages of inflammation, necrosis, and regeneration (see Images 1-2). Findings include focal endomysial infiltration by mononuclear cells (consisting of mostly CD8⁺ T lymphocytes and macrophages), capillary obliteration, endothelial cell damage, and increased amounts of connective tissue. Later in the course of PM, muscle-cell degeneration, fibrosis, and regeneration may be observed. IBM is histologically similar to PM with the additional presence of intracytoplasmic inclusion bodies observed on electron microscopy. DM shows inflammatory changes, predominantly in the perimysial and perivascular regions with CD4⁺ T and B lymphocytes. Corticosteroid-induced myopathy causes no inflammatory changes. Type II fiber atrophy is the characteristic feature.

TREATMENT

Section 6 of 11  

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• Treatment
• Medication
• Follow-up
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Medical Care

Treatment of PM is empirical because of the rarity of the disease and the paucity of randomized controlled trials.

• Prednisone is the first-line treatment of choice for PM.
• • Typically, the dose is 1 mg/kg/d, either as a single dose or divided. This high dose is usually continued for 4-8 weeks, until the CK level returns to reference ranges. Taper prednisone on a monthly basis by 5-10 mg until the lowest dose that controls the disease is reached.
  • Monitor response to therapy by improvement in muscle strength and muscle endurance and by decrease in CK levels.
  • Closely monitor patients for disease activity and adverse effects of corticosteroids such as weight gain, hypertension, osteopenia, and steroid myopathy.
  • Corticosteroid myopathy can occur during the course of treatment and must be distinguished from reactivation of muscle disease. CK level is usually within reference ranges in cases of steroid myopathy. No improvement is observed with raised doses of steroids, and the condition worsens if the dose is increased.
• Immunosuppressive agents are indicated if patients do not show improvement with steroids within a reasonable period (ie, 4 wk) or if adverse effects from corticosteroids develop. Patients with poor prognostic indicators, such as dysphagia or dysphonia, are likely to require immunosuppressive agents. Under these circumstances, methotrexate is the second-line agent. Patients with IBM usually respond poorly to corticosteroids and immunosuppressive agents.
• • Obtain baseline liver function tests and pulmonary functions before initiating therapy.
  • Azathioprine, cyclophosphamide, chlorambucil, and cyclosporine have been used with varying success as second-line agents for PM.
• Intravenous immunoglobulin (IVIG) has been used for the short-term treatment of steroid-resistant cases of PM.
• The role of newer agents, such as TNF inhibitors, remains unclear. However, the use of TNF inhibitors in refractory cases has demonstrated some success.
• Recently, an open-label study of patients with dermatomyositis treated with rituximab (anti-CD20 monoclonal antibody) provided encouraging results. This may be a new approach to therapy for refractory cases.
• Extramuscular manifestations
• • Constitutional symptoms, such as fever and fatigue, usually respond to corticosteroids.
  • Articular symptoms usually resolve with treatment of the myositis. Occasionally, patients may develop a rheumatoidlike arthropathy, which may require immunosuppressive treatment such as methotrexate.
  • Patients with severe interstitial lung disease may benefit from high-dose steroids and immunosuppressive treatment, especially cyclophosphamide.
  • Cardiac abnormalities may respond to corticosteroids. Symptomatic arrhythmias require antiarrhythmic therapy, and symptomatic heart block is treated with placement of a pacemaker.
  • Dysphagia responds either slowly or poorly to immunosuppressive therapies and may be severe enough to require enteral feeding through a gastrostomy tube or parenteral nutrition.

Consultations

• Rheumatologists
• Neurologists
• Pulmonary specialists
• Cardiologists
• Physical therapists
• Speech therapists (for swallow evaluation)

Diet

• Patients may benefit from a high-protein diet. Histamine 2 receptor antagonists, proton pump inhibitors, and/or prokinetic agents may be useful in patients with esophageal reflux and dysmotility.
• Monitor patients to avoid excessive weight gain due to corticosteroid use.
• Prescribe calcium with vitamin D supplementation and oral bisphosphonates for osteoporosis prophylaxis.

Activity

• Encourage patients to start a supervised exercise program early in the course of the disease.
• During the acute stage of the disease, patients may benefit from heat therapy, passive range of motion exercises, and splints to avoid contractures.
• Once acute inflammation is under control, the rehabilitation program should include active range-of-motion exercises and isometric contractions of the muscle groups.
• With improvement in muscle strength, patients should perform isotonic exercises with light resistance.
• Encourage patients to do 15-30 minutes of aerobic exercise when the disease is inactive.

MEDICATION

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The goals of pharmacotherapy are to reduce morbidity and to prevent complications.

Drug Category: Corticosteroids

Inhibit the inflammatory process by multiple mechanisms, including inhibiting proinflammatory cytokine production, monocyte/macrophage function, and angiogenesis.

Drug Category: Immunosuppressants

May be of benefit for patients whose conditions have not responded to steroids or for patients unable to tolerate prednisone.

Drug Category: Tumor necrosis factor inhibitors

May be used in refractory cases of PM that have failed to respond to conventional therapy with steroids.

FOLLOW-UP

Section 8 of 11  

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• Introduction
• Clinical
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• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Further Inpatient Care

• Closely monitor patients in the hospital while they are taking high-dose corticosteroids.
• Monitor serial CK levels to assess improvement.
• Severe pulmonary or cardiac involvement may require management in an intensive care setting.
• Ideally, purified protein derivative should be placed prior to initiation of corticosteroid treatment.
• Regularly monitor CBC counts, liver function test findings, and renal function in patients treated with immunosuppressive agents.
• Patients usually need aggressive inpatient physical therapy.

Further Outpatient Care

• See patients every 2-3 weeks initially; if they are stable, see them at monthly intervals thereafter.
• Frequently check laboratory tests, including CK (as outlined in Lab Studies), and document muscle strength evaluation results.
• Check patients' weight with each visit.
• Routine age-appropriate cancer screening is recommended.
• Arrange outpatient physical therapy.

Deterrence/Prevention

• Avoid cold exposure if Raynaud phenomenon is a significant problem.
• Patients with esophageal involvement can elevate the head of the bed and avoid eating before bedtime to minimize reflux and risk of aspiration.

Complications

• Pulmonary disease
• • Interstitial lung disease
  • Aspiration pneumonia
• Cardiac
• • Heart block
  • Arrhythmias
  • Congestive heart failure
  • Pericarditis
• Gastrointestinal
• • Dysphagia
  • Malabsorption
• Malignancy: Incidence of lung, bladder, and non-Hodgkin lymphoma may be increased in patients with PM, especially in the first year after diagnosis.

Prognosis

• Most patients' conditions respond well to treatment, although residual weakness is common. Osteoporosis, a common complication of chronic corticosteroid therapy, may cause significant morbidity.
• Poor prognostic factors include the following:
• • Older age
  • Female sex
  • African American race
  • Interstitial lung disease
  • Presence of anti-Jo-1 (lung disease) and anti-SRP antibodies (severe muscle disease, cardiac involvement)
  • Associated malignancy
  • Delayed or inadequate treatment
  • Dysphagia, dysphonia
  • Cardiac and pulmonary involvement

Patient Education

• Educate patients early about the disease, and provide them with realistic expectations about outcomes. Most patients show significant improvement with treatment.
• • Stress the need for close follow-up care, continued physical therapy, and long-term therapy.
  • Warn patients regarding adverse events related to medications.
  • Patients may visit the Myositis Association of America Web site for more information.
• For excellent patient education resources, visit eMedicine's Muscle Disorders Center. Also, see eMedicine's patient education article Chronic Fatigue Syndrome.

MISCELLANEOUS

Section 9 of 11  

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• Clinical
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• Follow-up
• Miscellaneous
• Multimedia
• References

Medical/Legal Pitfalls

• Delay in diagnosis
• Misdiagnosis
• Failure to diagnose malignancy
• Failure to inform patient about potential drug toxicity

MULTIMEDIA

Section 10 of 11  

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• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

Media file 1:  MRI of thighs showing increased signal in the quadriceps muscles bilaterally consistent with inflammatory myositis.
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Media type:  Photo

Media file 2:  Histopathology of polymyositis showing endomysial mononuclear inflammatory infiltrate and muscle fiber necrosis.
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Media type:  Photo

Media file 3:  Close view of muscle biopsy, showing chronic inflammatory infiltrate consisting of T lymphocytes, especially CD8+ T lymphocytes.
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Media type:  Photo

REFERENCES

Section 11 of 11

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
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• Follow-up
• Miscellaneous
• Multimedia
• References

• Adams EM, Plotz PH. The treatment of myositis. How to approach resistant disease. Rheum Dis Clin North Am. Feb 1995;21(1):179-202. [Medline].
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• Alexanderson H. Exercise: an important component of treatment in the idiopathic inflammatory myopathies. Curr Rheum Rep. Apr 2005;7(2):115-24.
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• Hicks JE. Rehabilitating patients with idiopathic inflammatory myopathy. Musculoskel Med. 1995;12:4-54.
• Hill CL, Zhang Y, Sigurgeirsson B, et al. Frequency of specific cancer types in dermatomyositis and polymyositis: a population-based study. Lancet. Jan 13 2001;357(9250):96-100. [Medline].
• Kumar A, Teuber SS, Gershwin ME. Intravenous immunoglobulin: striving for appropriate use. Int Arch Allergy Immunol. 2006;140(3):185-98.
• Marie I, Hachulla E, Cherin P, et al. Opportunistic infections in polymyositis and dermatomyositis. Arthritis Rheum. Apr 15 2005;53(2):155-65. [Medline].
• O''Rourke KS. Myopathies in the elderly. Rheum Dis Clin North Am. Aug 2000;26(3):647-72, viii. [Medline].
• Oddis CV. Idiopathic inflammatory myopathies: a treatment update. Curr Rheumatol Rep. Dec 2003;5(6):431-6. [Medline].
• Plotz PH, Miller F, Hoffman E, et al. Workshop on inflammatory myopathy Bethesda, 5-6 April 2000. Neuromuscul Disord. Jan 2001;11(1):93-5. [Medline].
• Plotz PH, Rider LG, Targoff IN, et al. NIH conference. Myositis: immunologic contributions to understanding cause, pathogenesis, and therapy. Ann Intern Med. May 1 1995;122(9):715-24. [Medline].
• Reimers CD, Finkenstaedt M. Muscle imaging in inflammatory myopathies. Curr Opin Rheumatol. Nov 1997;9(6):475-85. [Medline].
• Saadeh CK. Entanercept is effective in the treatment of polymyositis/dermatomyositis which is refractory to conventional therapy including steroids and other disease modifying agents. Arthritis and Rheumatism. 2000;43(9):S193.
• Salomonsson S, Lundberg IE. Cytokines in Idiopathic Inflammatory Myopathies. Autoimmunity. May 2006;39 (3):177-90.
• Schnabel A, Hellmich B, Gross WL. Interstitial lung disease in polymyositis and dermatomyositis. Curr Rheumatol Rep. Apr 2005;7(2):99-105. [Medline].
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Article Last Updated: Aug 25, 2006